docs: def check_grid_coordinates(self, ds):
          """
          5.6 When the coordinate variables for a horizontal grid are not
          longitude and latitude, it is required that the true latitude and
          longitude coordinates be supplied via the coordinates attribute.
  
          :param netCDF4.Dataset ds: An open netCDF dataset
          :rtype: list
          :return: List of results
          """
          ret_val = []
          latitudes = cfutil.get_true_latitude_variables(ds)
          longitudes = cfutil.get_true_longitude_variables(ds)
  
          check_featues = [
              '2d-regular-grid',
              '2d-static-grid',
              '3d-regular-grid',
              '3d-static-grid',
              'mapped-grid',
              'reduced-grid'
          ]
  
          # This one is tricky because there's a very subtle difference between
          # latitude as defined in Chapter 4 and "true" latitude as defined in
          # chapter 5.
  
          # For each geophysical variable that defines a grid, assert it is
          # associated with a true latitude or longitude coordinate.
  
          for variable in self._find_geophysical_vars(ds):
              # We use a set so we can do set-wise comparisons with coordinate
              # dimensions
              dimensions = set(ds.variables[variable].dimensions)
              # If it's not a grid, skip it
              if cfutil.guess_feature_type(ds, variable) not in check_featues:
                  continue
              has_coords = TestCtx(BaseCheck.HIGH, self.section_titles['5.6'])
  
              # axis_map is a defaultdict(list) mapping the axis to a list of
              # coordinate names. For example:
              # {'X': ['lon'], 'Y':['lat'], 'Z':['lev']}
              # The mapping comes from the dimensions of the variable and the
              # contents of the `coordinates` attribute only.
              axis_map = cfutil.get_axis_map(ds, variable)
  
              msg = '{}\'s coordinate variable "{}" is not one of the variables identifying true '+\
                    'latitude/longitude and its dimensions are not a subset of {}\'s dimensions'
  
              alt = '{} has no coordinate associated with a variable identified as true latitude/longitude; '+\
                    'its coordinate variable should also share a subset of {}\'s dimensions'
  
              # Make sure we can find latitude and its dimensions are a subset
              _lat = None
              found_lat = False
              for lat in axis_map['Y']:
                  _lat = lat
                  is_subset_dims = set(ds.variables[lat].dimensions).issubset(dimensions)
  
                  if is_subset_dims and lat in latitudes:
                      found_lat = True
                      break
              if _lat:
                  has_coords.assert_true(found_lat, msg.format(variable, _lat, variable))
              else:
                  has_coords.assert_true(found_lat, alt.format(variable, variable))
  
  
              # Make sure we can find longitude and its dimensions are a subset
              _lon = None
              found_lon = False
              for lon in axis_map['X']:
                  _lon = lon
                  is_subset_dims = set(ds.variables[lon].dimensions).issubset(dimensions)
  
                  if is_subset_dims and lon in longitudes:
                      found_lon = True
                      break
              if _lon:
                  has_coords.assert_true(found_lon, msg.format(variable, _lon, variable))
              else:
                  has_coords.assert_true(found_lon, alt.format(variable, variable))
  
              ret_val.append(has_coords.to_result())
          return ret_val